生物谷報道：美國密歇根州大學(xué)科學(xué)家近日在《美國國家科學(xué)院院刊》上發(fā)表論文,，通過對大腸桿菌的研究發(fā)現(xiàn)這種細(xì)菌在實驗室進化出新的特點,，并可通過傳代“重演”進化史。
20年前,，美國密歇根州大學(xué)進化生物學(xué)家理查德·倫斯基取單個大腸桿菌繁殖,，讓其后代繁衍了12支實驗室細(xì)菌種群。如今當(dāng)倫斯基觀察它們發(fā)生了什么情況時,，發(fā)現(xiàn)這12支細(xì)菌種群還在成長壯大,，并逐漸積累了變異,，繁殖出了4.4萬多代細(xì)菌。在每一個獨立的種群中,，倫斯基看到大多數(shù)細(xì)菌的模樣是相似的,。比如，所有12支細(xì)菌種群都進化出了更大的細(xì)胞,、更快的生長速度和較低的高峰種群密度,。
然而有一些特點與眾不同，大約是3.15萬代細(xì)菌出現(xiàn)了戲劇性的變化,，突然獲得了代謝檸檬酸鹽的能力,，而檸檬酸鹽是其培養(yǎng)基中的第二種營養(yǎng)物質(zhì)，大腸桿菌通常利用不了它們,。事實上,，無能力利用檸檬酸鹽是細(xì)菌學(xué)家區(qū)分大腸桿菌和其它細(xì)菌的特征之一。而這種利用檸檬酸鹽的變異種群增加了大腸桿菌的種群數(shù)量和多樣性,。倫斯基說：“這是我們做實驗時看到的最深刻的變化,。顯然有些細(xì)菌完全與眾不同，甚至不能考慮為正常的大腸桿菌,，這使它特別有趣味,。”
到現(xiàn)在，倫斯基統(tǒng)計過足夠的細(xì)菌細(xì)胞,，發(fā)現(xiàn)其中所有樣本的突變都是幾次積累后實現(xiàn)的,。這意味著利用檸檬酸鹽的能力很特別，可能是一種罕見的不可能的變異種類,，或一種罕見的染色體倒置或是幾次基因排序突變的累積效果,。為發(fā)現(xiàn)是種什么樣的變異，倫斯基轉(zhuǎn)向冰柜中保存的每種種群每500代的樣本,，讓它們再現(xiàn)他所需要的進化歷史,。其辦法是讓細(xì)菌蘇醒，再讓它們重演一次進化歷程,。
同一種群會再次進化出能利用檸檬酸鹽的能力嗎,？他想總會有一種群會出現(xiàn)這種累積效果的。結(jié)果果然不出所料,。這種重演表明原有種群在2萬代或以上時開始出現(xiàn)了這種進化重演,。他總結(jié)在大約2萬代時一定突然發(fā)生了一些事情，從而使細(xì)菌進化出這種利用檸檬酸鹽的能力,。
倫斯基及其同事正在識別一些細(xì)菌的較早變化,，以了解這種變異如何在1萬代之后的細(xì)菌中產(chǎn)生的。與此同時，實驗表明進化并不總是朝著最好的結(jié)果出現(xiàn),。相反一些變化事件有時會開放一種種群的進化之門,，有時則會關(guān)閉其它種群的進化之門。這對反對進化論的人給了反駁一擊,。（生物谷www.bioon.com）
生物谷推薦原始出處：
PNAS | June 10, 2008 | vol. 105 | no. 23 | 7899-7906,DOI,10.1073/pnas.0803151105
Historical contingency and the evolution of a key innovation in an experimental population of Escherichia coli
Zachary D. Blount, Christina Z. Borland, and Richard E. Lenski*
Department of Microbiology and Molecular Genetics, Michigan State University, East Lansing, MI 48824
Contributed by Richard E. Lenski, April 9, 2008 (received for review March 26, 2008)
The role of historical contingency in evolution has been much debated, but rarely tested. Twelve initially identical populations of Escherichia coli were founded in 1988 to investigate this issue. They have since evolved in a glucose-limited medium that also contains citrate, which E. coli cannot use as a carbon source under oxic conditions. No population evolved the capacity to exploit citrate for >30,000 generations, although each population tested billions of mutations. A citrate-using (Cit+) variant finally evolved in one population by 31,500 generations, causing an increase in population size and diversity. The long-delayed and unique evolution of this function might indicate the involvement of some extremely rare mutation. Alternately, it may involve an ordinary mutation, but one whose physical occurrence or phenotypic expression is contingent on prior mutations in that population. We tested these hypotheses in experiments that "replayed" evolution from different points in that population's history. We observed no Cit+ mutants among 8.4 x 1012 ancestral cells, nor among 9 x 1012 cells from 60 clones sampled in the first 15,000 generations. However, we observed a significantly greater tendency for later clones to evolve Cit+, indicating that some potentiating mutation arose by 20,000 generations. This potentiating change increased the mutation rate to Cit+ but did not cause generalized hypermutability. Thus, the evolution of this phenotype was contingent on the particular history of that population. More generally, we suggest that historical contingency is especially important when it facilitates the evolution of key innovations that are not easily evolved by gradual, cumulative selection.
